Gypsum products utilizing a two-repeating unit dispersant and a method for making them

ABSTRACT

A gypsum slurry includes water, a hydraulic component including stucco and a polycarboxylate dispersant. The dispersant has two repeating units wherein the first repeating unit is an olefinic unsaturated mono-carboxylic acid repeating unit or an ester or salt, and the second repeating unit is a vinyl or allyl group bound to a polyether by an ether linkage. The slurry can be made into a gypsum panel.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. Ser. No.11/449,924 entitled “Gypsum Products Utilizing a Two-Repeating UnitDispersant and a Method of Making Them”, filed Jun. 9, 2006, which is acontinuation-in-part of U.S. Ser. No. 11/152,418 entitled “GypsumProducts Utilizing a Two-Repeating Unit Dispersant and a Method ofMaking Them”, filed Jun. 14, 2005, now abandoned, both hereinincorporated by reference.

This application is further related to U.S. Ser. No. 11/450,122(Attorney Ref. No. 2033.75341), entitled, “Effective Use of Dispersantsin Wallboard Containing Foam”, filed Jun. 9, 2006 and herebyincorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates to fast-drying gypsum products. Morespecifically, it relates to a gypsum slurry and a wallboard thatrequires less drying time or less energy than conventional products.

Gypsum-based building products are commonly used in construction.Wallboard made of gypsum is fire retardant and can be used in theconstruction of walls of almost any shape. It is used primarily as aninterior wall and ceiling product. Gypsum has sound-deadeningproperties. It is relatively easily patched or replaced if it becomesdamaged. There are a variety of decorative finishes that can be appliedto the wallboard, including paint and wallpaper. Even with all of theseadvantages, it is still a relatively inexpensive building material.

One reason for the low cost of wallboard panels is that they aremanufactured by a process that is fast and efficient. A slurry,including calcium sulfate hemihydrate and water, is used to form thecore, and is continuously deposited on a paper cover sheet movingbeneath a mixer. A second paper cover sheet is applied thereover and theresultant assembly is formed into the shape of a panel. Calcium sulfatehemihydrate reacts with a sufficient amount of the water to convert thehemihydrate into a matrix of interlocking calcium sulfate dihydratecrystals, causing it to set and to become firm. The continuous stripthus formed is conveyed on a belt until the calcined gypsum is set, andthe strip is thereafter cut to form boards of desired length, whichboards are conveyed through a drying kiln to remove excess moisture.Since each of these steps takes only minutes, small changes in any ofthe process steps can lead to gross inefficiencies in the manufacturingprocess.

The amount of water added to form the slurry is in excess of that neededto complete the hydration reaction. Some of the water that is added tothe gypsum slurry is used to hydrate the calcined gypsum, also known ascalcium sulfate hemihydrate, to form an interlocking matrix of calciumsulfate dihydrate crystals. Excess water gives the slurry sufficientfluidity to flow out of the mixer and onto the facing material to beshaped to an appropriate width and thickness. While the product is wet,it is very heavy to move and relatively fragile. The excess water isremoved from the board by evaporation. If the excess water were allowedto evaporate at room temperature, it would take a great deal of space tostack and store wallboard while it was allowed to air dry or to have aconveyor long enough to provide adequate drying time. Until the board isset and relatively dry, it is somewhat fragile, so it must be protectedfrom being crushed or damaged.

To dry the boards in a relatively short period of time, the wallboardproduct is usually dried by evaporating the extra water at elevatedtemperatures, for example, in an oven or kiln. It is relativelyexpensive to build and operate the kiln at elevated temperatures,particularly when the cost of fossil fuels rises. A reduction inproduction costs could be realized by reducing the amount of excesswater present in set gypsum boards that is later removed by evaporation.

Another reason to decrease water is that the strength of gypsum productsis inversely proportional to the amount of water used in itsmanufacture, especially in full density slurries. As the excess waterevaporates, it leaves voids in the matrix once occupied by the water.Where large amounts of water were used to fluidize the gypsum slurry,more and larger voids remain in the product when it is completely dry.These voids decrease the product density and strength in the finishedproduct.

Dispersants are known for use with gypsum that help fluidize the mixtureof water and calcium sulfate hemihydrate so that less water is needed tomake a flowable slurry. Naphthalene sulfonate dispersants are wellknown, but have limited efficacy. Polycarboxylate dispersants arecommonly used with cements and, to a lesser degree, with gypsum. Theclass of compounds represented by the term “polycarboxylate dispersants”is huge, and it is very difficult to predict how individual compoundsreact in different media.

Despite the large amount of prior art to polycarboxylate dispersants, itis difficult to predict the effect of any particular compound on theproducts with which it is used. Polycarboxylates are generally known toimprove fluidity in cement. This does not necessarily mean that everypolycarboxylate will produce the same result in gypsum products. Gypsumand cement form different crystal patterns that may disperse differentlyin a polycarboxylate solution. The set times of these hydraulicmaterials is very different, making the retardive effects of somepolycarboxylates that are negligible in cement critical to the set of agypsum wallboard. There are even variations within the realm of gypsumproducts, with some polycarboxylates being effective for certain gypsumsources and not others. The lack of predictability of polycarboxylateefficacy in gypsum or cement makes it difficult to make a low-waterwallboard product given the constraints of the manufacturing process.

Another disadvantage is that polycarboxylates are known to interact withother additives in gypsum products. For example, foam may be added togypsum board to reduce the weight of the board. However, somepolycarboxylates destabilize some foams, causing it to collapse and loseits effectiveness before the board sets. Reaction of the polycarboxylatewith foam is not generally predictable from knowledge of the chemicalstructure of the particular polycarboxylate dispersant being used.

In addition to acting as a dispersant, polycarboxylates are known toretard the set of hydraulic slurries. Retardation in the set of a cementslurry by several minutes would be negligible. However, on a high-speedwallboard line, set retardation of minutes could result in board toosoft to cut, too fragile to move to the kiln or even too soft to conveyon the production line. Wallboard should be approximately 50% set whenit is cut at the knife in order to withstand subsequent handling. Whenhigh dosages of polycarboxylates are used to make a flowable slurry atlow water to stucco ratios, the set time can be delayed enough torequire reduction in the speed of the board making line, severelyreducing efficiency. Thus, retardation of the product set times of onlyminutes has the potential to reduce the productivity of a board line byhalf, while such a delay in the processing of cementitious materialswould not be noticed.

Further, retardation of the set times by the use of polycarboxylatedispersants cannot always be overcome by the addition of setaccelerators. The addition of set accelerators to the mixer decreasesthe set time, but also causes the formation of dihydrate crystals beforethe slurry leaves the mixer, resulting in premature thickening andreduced flowability of the slurry. Thus, the use of set accelerators toovercome the set retardation can defeat the purpose of adding thepolycarboxylate dispersant to increase flowability in the first place.

It would be a great improvement if a gypsum slurry were developed fromwhich wallboard could be made that did not require kiln drying or longdrying times. Moreover, the improved slurry would dry quickly withoutthe increase in set times associated with the use of polycarboxylatedispersants.

SUMMARY OF THE INVENTION

This and other needs are improved by the present invention of animproved gypsum slurry and wallboard product, and method of making them.The gypsum slurry includes water, a hydraulic component including atleast 50% calcium sulfate hemihydrate based on the weight of thehydraulic component and a specific two-repeating unit polycarboxylatedispersant.

The polycarboxylate dispersant is a co-polymer that includes a first anda second repeating unit, wherein said first repeating unit is anolefinic unsaturated mono-carboxylic acid repeating unit or an ester orsalt thereof, or an olefinic unsaturated sulphuric acid repeating unitor a salt thereof, and said second repeating unit is of the generalformula:

where R¹ is represented by

R² is hydrogen or an aliphatic C₁ to C₅ hydrocarbon group. R³ is anon-substituted or substituted aryl group, preferably phenyl. R⁴ ishydrogen or an aliphatic C₁ to C₂₀ hydrocarbon group, a cycloaliphaticC₅ to C₈ hydrocarbon group, a substituted C₆ to C₁₄ aryl group or agroup conforming to the formula

R⁵ and R⁷, independently of each other, represent an alkyl, aryl,aralkyl or alkylaryl group and R⁶ is a divalent alkyl, aryl, aralkyl oralkaryl group. p is 0 to 3, inclusive. m and n are, independently, aninteger from 2 to 4, inclusive. x and y are, independently, integersfrom 55 to 350, inclusive. The value of z is from 0 to 200, inclusive.

A second aspect of this invention is a gypsum panel that includes a coreof calcium sulfate dihydrate and the dispersant described above.

The gypsum slurry of this invention, the method of making it and thegypsum panel made therefrom result in a cost savings in the fuel burdenof the ovens. Less water needs to be driven from the pores of the gypsumproducts, allowing for a reduction in the oven temperature or the amountof time that the products spend in the oven. Fossil fuels are conservedand the cost savings therefrom can be realized.

The dispersant used in this invention is also less retardive than otherpolycarboxylate dispersants for the same fluidity. This reduces the needfor set accelerators and associated costs. It also allows the water tostucco ratio to be further reduced in a gypsum board before the greenstrength of the product is insufficient to continue manufacturing.

DETAILED DESCRIPTION OF THE INVENTION

A gypsum slurry is made from calcium sulfate hemihydrate, water and aspecific two-repeating unit dispersant. The dispersant includes acarboxylic acid repeating unit and an alkenyl polyether glycol repeatingunit.

The hydraulic material includes any calcium sulfate hemihydrate, alsoknown as stucco or calcined gypsum, preferably in amounts of at least50%. Preferably, the amount of calcium sulfate hemihydrate is at lest75%, at least 80% or at least 85% stucco. In many wallboardformulations, the hydraulic material is substantially all calciumsulfate hemihydrate. Any form of calcined gypsum may be used, includingbut not limited to alpha or beta stucco. Use of calcium sulfateanhydrite, synthetic gypsum or landplaster is also contemplated,although preferably in small amounts of less than 20%. Other hydraulicmaterials, including cement and fly ash, are optionally included in theslurry.

Although any stucco benefits from this invention, stuccos from differentsources include different amounts and types of salt and impurities. Theslurry of this invention is less effective when the calcium sulfatehemihydrate has relatively high concentrations of naturally occurringsalts. Low-salt stuccos are defined as those having soluble salts ofless than 300 parts per million. High salt content stuccos include thosehaving at least 600 parts per million soluble salts. Gypsum depositsfrom Southard, Okla., Little Narrows, Nova Scotia, Fort Dodge, Iowa,Sweetwater, Tex., Plaster City, Calif. and many other locations meetthis preference.

The dispersant used in the slurry includes two repeating units. Thefirst repeating unit is an olefinic unsaturated mono-carboxylic acidrepeating unit, an ester or salt thereof, or an olefinic unsaturatedsulphuric acid repeating unit or a salt thereof. Examples of the firstrepeating unit are acrylic acid, methacrylic acid, crotonic acid,isocrotonic acid, allyl sulfonic acid and vinyl sulfonic acid. Mono- ordivalent salts are suitable in place of the hydrogen of the acid group.The hydrogen can also be replaced by hydrocarbon group to form theester. Preferred repeating units include acrylic acid or methacrylicacid.

The second repeating unit satisfies Formula I,

and R¹ is derived from an unsaturated (poly)alkylene glycol ether groupaccording to Formula II.

Referring to Formulas I and II, the alkenyl repeating unit optionallyincludes a C₁ to C₃ alkyl group between the polymer backbone and theether linkage. The value of p is an integer from 0-3, inclusive.Preferably, p is either 0 or 1. R² is either a hydrogen atom or analiphatic C₁ to C₅ hydrocarbon group, which may be linear, branched,saturated or unsaturated. R³ is a non-substituted or substituted arylgroup, preferably phenyl. Examples of preferred repeating units includeacrylic acid and methacrylic acid.

The polyether group of Formula II contains multiple C₂-C₄ alkyl groups,including at least two alkyl groups, connected by oxygen atoms. m and nare, independently, integers from 2 to 4, inclusive, preferably, atleast one of m and n is 2. x and y are, independently, integers from 55to 350, inclusive. The value of z is from 0 to 200, inclusive. R⁴ ishydrogen or an aliphatic C₁ to C₂₀ hydrocarbon group, a cycloaliphaticC₅ to C₈ hydrocarbon group, a substituted C₆ to C₁₄ aryl group or agroup conforming at least one of Formula III(a), III(b) and III(c).

In the above formulas, R⁵ and R⁷, independently of each other, representan alkyl, aryl, aralkyl or alkylaryl group. R⁶ is a bivalent alkyl,aryl, aralkyl or alkylaryl group.

A particularly useful dispersant of this group, referenced as the“PCE211-Type” dispersant, is designated PCE211 (hereafter “211”). Otherpolymers in this series known to be useful in wallboard include PCE111.This class of dispersants and how to make them is further described inU.S. Ser. No. 11/152,678, entitled “Polyether-Containing Copolymer”,filed Jun. 14, 2005 and herein incorporated by reference.

The molecular weight of the dispersant is preferably from about 20,000to about 60,000 Daltons. Surprisingly, it has been found that the lowermolecular weight dispersants cause less retardation of set time thandispersants having a molecular weight greater than 60,000 Daltons.Generally longer side chain length, which results in an increase inoverall molecular weight, provides better dispersibility. However, testswith gypsum indicate that efficacy of the dispersant is reduced atmolecular weights above 60,000 Daltons.

Many polymers can be made with the same two repeating units usingdifferent distributions of them. The ratio of the acid-containingrepeating units to the polyether-containing repeating unit is directlyrelated to the charge density. Preferably, the charge density of theco-polymer is in the range of about 300 to about 3000 μequiv. charges/gco-polymer. It has been found that the most effective dispersant testedfor water reduction in this class of dispersants, MELFLUX 2651F, has thehighest charge density. MELFLUX dispersants are manufactured by DegussaConstruction Polymers, GmbH, Trostberg, Germany, and marketed in theU.S. by Degussa Corporation, Kennesaw, Ga., hereafter “Degussa”.(MELFLUX is a registered trademark of Degussa Construction Polymers,GmbH.)

However, it has also been discovered that the increase in charge densityfurther results in an increase in the retardive effect of thedispersant. Dispersants with a low charge density, such as MELFLUX2500L, retard the set times less than the MELFLUX 2651F dispersant thathas a high charge density. Since retardation in set times increases withthe increase in efficacy obtained with dispersants of high chargedensity, making a slurry with low water, good flowability and reasonableset times requires keeping of the charge density in a mid-range. Morepreferably, the charge density of the co-polymer is in the range ofabout 600 to about 2000 μequiv. charges/g co-polymer.

This dispersant is particularly well-suited for use with gypsum. Whilenot wishing to be bound by theory, it is believed that the acidrepeating units bind to the hemihydrate crystals while the longpolyether chains of the second repeating unit on the backbone performthe dispersing function. Balancing of the length of the polyetherchains, the total molecular weight and the charge density are importantfactors in designing a dispersant for gypsum. Since it is less retardivethan other disperants, it is less disruptive to the manufacturingprocess of gypsum products. The dispersant is used in any effectiveamount. To a large extent, the amount of dispersant selected isdependent on the desired fluidity of the slurry. As the amount of waterdecreases, more dispersant is required to maintain a constant slurryfluidity. Preferably, the dispersant is used in amounts of about 0.01%to about 0.5% based on the dry weight of the stucco. More preferably,the dispersant is used in amounts of about 0.05% to about 0.2% on thesame basis. In measuring a liquid dispersant, only the polymer solidsare considered in calculating the dosage of the dispersant, and thewater from the dispersant is considered when a water/stucco ratio iscalculated. This dispersant allows for the design of a high-speedwallboard manufacturing process where the board is at least 50% setwithin five minutes. Even in the absence of accelerators, at least 50%set is achievable within ten minutes.

Another factor that may be important in the selection of dispersantconcentration is the bond with the facing material. Some papers requirean additional component to give a satisfactory bond at high dispersantdosages. A different bonding system, such as polyvinyl alcohol, isuseful. Another technique that aids in the paper bond is the applicationof a set accelerator, such as alum, to the paper to speed setting of theslurry onto the facing material.

Polymerization of the repeating units to make the copolymer dispersantis carried out by any method known by an artisan. Preferredpolymerization techniques are taught in U.S. Ser. No. 11/152,678,entitled “Polyether-Containing Copolymer”, filed Jun. 14, 2005,previously incorporated by reference.

Some embodiments of this invention include combination of the dispersantcopolymer with a defoaming component. This alternative may be realizedby an addition of the dispersant and the defoaming agent to theconstruction material composition in separated form depending from thespecific application. In another embodiment, the copolymer includes thedefoaming component as a third repeating unit in copolymerized form.

Third repeating units, such as styrene or acrylamides, may beadditionally co-polymerized with the first and second repeating units.Alternatively, there also may be used components with hydrophobicproperties. Compounds with ester structural units, polypropylene oxideor polyethylene oxide (PO/PE)-units are also used. These third repeatingunits should be represented in the copolymer in amounts up to 5 mol %;amounts from 0.05 to 3 mol % and 0.1 to 1.0 mol % are preferred.

In the case where the defoaming component is added to the composition inseparated form, representatives of the following group are to be seen aspreferred defoamers: non-ionic tensides like copolymers comprisingethylene oxide/propylene oxide-(EO-PO)-units (Dowfax™ of the Dowcompany) or EO-PO-EO or PO-EO-PO block copolymers, respectively(Pluronic™ of BASF). Additionally, mineral oil based defoamers can alsobe used. Defoamers can be used in powder form such an Agitan types ofthe Munzing Chemie Company.

Water is added to the slurry in any amount that makes a flowable slurry.The amount of water to be used varies greatly according to theapplication with which it is being used, the exact dispersant beingused, the properties of the stucco and the additives being used. Thewater to stucco ratio (“WSR”) with wallboard is preferably about 0.1 toabout 0.8 based on the dry weight of the stucco. Commonly a WSR of about0.2 to about 0.6 is preferred. Flooring compositions preferably use aWSR from about 0.17 to about 0.45, preferably from about 0.17 to about0.34. Moldable or castable products preferably use water in a WSR offrom about 0.1 to about 0.3, preferably from about 0.16 to about 0.25.The WSR can be reduced to 0.1 or less in laboratory tests based on themoderate addition of the PCE211-Type dispersants.

Water used to make the slurry should be as pure as practical for bestcontrol of the properties of both the slurry and the set plaster. Saltsand organic compounds are well known to modify the set time of theslurry, varying widely from accelerators to set inhibitors. Someimpurities lead to irregularities in the structure as the interlockingmatrix of dihydrate crystals forms, reducing the strength of the setproduct. Product strength and consistency is thus enhanced by the use ofwater that is as contaminant-free as practical.

The gypsum slurry also optionally includes one or more modifiers thatenhance the ability of the polycarboxylate dispersant to fluidize theslurry, thus improving its efficacy. The two-repeating unit dispersantused here is particularly susceptible to the effects of the modifiers.Preferred modifiers include cement, lime, also known as quicklime orcalcium oxide, slaked lime, also known as calcium hydroxide, soda ash,also known a sodium carbonate, and other carbonates, silicates,phosphonates and phosphates. Dosage of the modifier is from 0.05% toabout 1% depending on the modifier being used and the application withwhich it is used. When modifiers are used, the efficacy of thedispersant is boosted to achieve a new level of fluidity, or the amountof polycarboxylate dispersant can be decreased to reduce thepolycarboxylate expense. Additional information on modifiers and theiruse is found in U.S. Ser. No. 11/152,317, entitled “Modifiers ForPolycarboxylate Dispersants”, previously incorporated by reference.

Modifiers have been found to be less effective when added to the slurryafter the dispersant contacts the calcined gypsum. Preferably themodifiers and the dispersant are added to the mixer water prior to theaddition of the hemihydrate. If both the modifier and the dispersant arein dry form, they can be preblended with each other and added with thestucco. This sequence of addition yields more of a boost in efficacy ofthe dispersant. A method for adding dispersants and modifiers to astucco composition is disclosed in more detail in co-pending U.S. Ser.No. 11/152,323, entitled “Method of Making a Gypsum Slurry withModifiers and Dispersants”, previously incorporated by reference.

Additional additives are also added to the slurry as are typical for theparticular application to which the gypsum slurry will be put. Setretarders (up to about 2 lb./MSF (9.8 g/m2)) or dry accelerators (up toabout 35 lb./MSF (170 g/m2)) are added to modify the rate at which thehydration reactions take place. “CSA” is a set accelerator comprising95% calcium sulfate dihydrate co-ground with 5% sugar and heated to 250°F. (121° C.) to caramelize the sugar. CSA is available from USGCorporation, Southard, Okla. plant, and is made according to U.S. Pat.No. 3,573,947, herein incorporated by reference. Potassium sulfate isanother preferred accelerator. HRA is calcium sulfate dihydrate freshlyground with sugar at a ratio of about 5 to 25 pounds of sugar per 100pounds of calcium sulfate dihydrate. It is further described in U.S.Pat. No. 2,078,199, herein incorporated by reference. Both of these arepreferred accelerators.

Another accelerator, known as wet gypsum accelerator or WGA, is also apreferred accelerator. A description of the use of and a method formaking wet gypsum accelerator are disclosed in U.S. Pat. No. 6,409,825,herein incorporated by reference. This accelerator includes at least oneadditive selected from the group consisting of an organic phosphoniccompound, a phosphate-containing compound or mixtures thereof. Thisparticular accelerator exhibits substantial longevity and maintains itseffectiveness over time such that the wet gypsum accelerator can bemade, stored, and even transported over long distances prior to use. Thewet gypsum accelerator is used in amounts ranging from about 5 to about80 pounds per thousand square feet (24.3 to 390 g/m²) of board product.

In some embodiments of the invention, additives are included in thegypsum slurry to modify one or more properties of the final product.Additives are used in the manner and amounts as are known in the art.Concentrations are reported in amounts per 1000 square feet of finishedboard panels (“MSF”). Starches are used in amounts from about 3 to about20 lbs./MSF (14.6 to 97.6 g/m²) to increase the paper bond andstrengthen the product. Glass fibers are optionally added to the slurryin amounts of at least 11 lb./MSF (54 g/m²). Up to 1.5 lb./MSF (73.2g/m²) of paper fibers are also added to the slurry. Wax emulsions areadded to the gypsum slurry in amounts up to 90 lb./MSF (0.4 kg/m²) toimprove the water-resistency of the finished gypsum board panel.

In embodiments of the invention that employ a foaming agent to yieldvoids in the set gypsum-containing product to provide lighter weight,any of the conventional foaming agents known to be useful in preparingfoamed set gypsum products can be employed. Many such foaming agents arewell known and readily available commercially, e.g. the HYONIC line ofsoap products from GEO Specialty Chemicals, Ambler, Pa. Foams and apreferred method for preparing foamed gypsum products are disclosed inU.S. Pat. No. 5,683,635, herein incorporated by reference. If foam isadded to the product, the polycarboxylate dispersant is optionallydivided between the gauge water and the foam water or two differentdispersants are used in the gauge water and the foam water prior to itsaddition to the calcium sulfate hemihydrate. This method is disclosed inco-pending application U.S. Ser. No. 11/152,404, entitled, “EffectiveUse of Dispersants in Wallboard Containing Foam”, previouslyincorporated by reference.

A trimetaphosphate compound is added to the gypsum slurry in someembodiments to enhance the strength of the product and to improve sagresistance of the set gypsum. Preferably the concentration of thetrimetaphosphate compound is from about 0.07% to about 2.0% based on theweight of the calcined gypsum. Gypsum compositions includingtrimetaphosphate compounds are disclosed in U.S. Pat. Nos. 6,342,284 and6,632,550, both herein incorporated by reference. Exemplarytrimetaphosphate salts include sodium, potassium or lithium salts oftrimetaphosphate, such as those available from Astaris, LLC., St. Louis,Mo. Care must be exercised when using trimetaphosphate with lime orother modifiers that raise the pH of the slurry. Above a pH of about9.5, the trimetaphosphate loses its ability to strengthen the productand the slurry becomes severely retardive.

Other potential additives to the wallboard are biocides to reduce growthof mold, mildew or fungi. Depending on the biocide selected and theintended use for the wallboard, the biocide can be added to thecovering, the gypsum core or both. Examples of biocides include boricacid, pyrithione salts and copper salts. Biocides can be added to eitherthe covering or the gypsum core. When used, biocides are used in thecoverings in amounts of less than 500 ppm.

In addition, the gypsum composition optionally can include a starch,such as a pregelatinized starch or an acid-modified starch. Theinclusion of the pregelatinized starch increases the strength of the setand dried gypsum cast and minimizes or avoids the risk of paperdelamination under conditions of increased moisture (e.g., with regardto elevated ratios of water to calcined gypsum). One of ordinary skillin the art will appreciate methods of pregelatinizing raw starch, suchas, for example, cooking raw starch in water at temperatures of at leastabout 185° F. (85° C.) or other methods. Suitable examples ofpregelatinized starch include, but are not limited to, PCF 1000 starch,commercially available from Lauhoff Grain Company and AMERIKOR 818 andHQM PREGEL starches, both commercially available from Archer DanielsMidland Company. If included, the pregelatinized starch is present inany suitable amount. For example, if included, the pregelatinized starchcan be added to the mixture used to form the set gypsum composition suchthat it is present in an amount of from about 0.5% to about 10% percentby weight of the set gypsum composition. Starches such as USG95 (UnitedStates Gypsum Company, Chicago, Ill.) are also optionally added for corestrength.

In operation, the gypsum is moved on a conveyor toward a mixer. Prior toentry into the mixer, dry additives, such as dry set accelerators, areadded to the powdered gypsum. Some additives are added directly to themixer via a separate line. Trimetaphosphate was added using this methodin the examples described below. Other additives may also be added tothe water. This is particularly convenient where the additives are inliquid form. For most additives, there is no criticality regardingplacing the additives in the slurry, and they may be added usingwhatever equipment or method is convenient.

However, when using the dispersant of this invention, it is important toadd the dispersant to the water prior to addition of the stucco. Gaugewater or make-up water is added at the mixer in amounts needed to meetthe target water to stucco ratio when water from other sources has beenconsidered.

Other known additives may be used as needed to modify specificproperties of the product. Sugars, such as dextrose, are used to improvethe paper bond at the ends of the boards. Wax emulsions or polysiloxanesare used for water resistance. If stiffness is needed, boric acid iscommonly added. Fire retardancy can be improved by the addition ofvermiculite. These and other known additives are useful in the presentslurry and wallboard formulations.

EXAMPLE 1

Tests were conducted to determine the effect of the addition ofpotassium carbonate on two different dispersants. In each of thefollowing samples, a gypsum slurry was made from 400 grams of stuccofrom Southard, Okla., 180 grams of water and 0.2% dispersant based onthe dry weight of the stucco. The 211 dispersant was made according toPreparation Example 3 of U.S. Ser. No. 11/152,678, filed Jun. 14, 2005,and U.S. Ser. No. 11/______ (Attorney Ref. DCP 3), both entitled“Polyether-Containing Copolymer” and hereby incorporated by reference.The dispersant type and amount of potassium carbonate are shown in TableI below, together with the results of the patty size and the stiffeningrate tests. TABLE I Potassium Patty Size, Stiffening DispersantCarbonate, g cm Time 211 0.6 30.3 6:00 211 0.0 19.8 2:05 MELFLUX 2500L0.6 26.0 10:30  Melflux 2500L 0.0 15.5 2:35

As is seen in the data in Table I above, the two-repeating unitdispersant of this invention, 211, shows reduced stiffening time bothwith and without the addition of a modifier, potassium carbonatecompared to a prior art dispersant, MELFLUX 2500L. This example alsodemonstrates the effect of the potassium carbonate modifier on each ofthese two dispersants. Note that the same concentration 211 dispersantresponds much better to the modifier, yielding a larger patty sizeindicating better dispersing action, but with a reduction in stiffeningtime.

EXAMPLE 2

The preferred 211 dispersant was tested with a variety of modifiers todetermine the improvement in efficacy. Reagent grade tetra sodiumphosphate (TSP), tetra sodium pyrophosphate (TSPP) and sodium carbonate(Soda Ash) were tested. DEQUEST 2006, a penta sodium salt of aminotri(methylene phosphonic acid), available from Solutia Inc., St. Louis,Mo., was also tested.

For all testing samples, the water to stucco ratio was 0.5 and 0.66% byweight wet gypsum accelerator (WGA) based on the dry weight of thestucco was added. The control sample had only 0.5% by weight WGA. Theamount of each modifier added is shown in Table II, along with the settime and patty size produced by each sample.

The modifier and dispersant were added to the water, followed byaddition of the stucco and WGA. The slurry was stirred until it wasconsistent. TABLE II DEQUEST Soda Modifier Control 2006 TSP TSPP AshAmount 0 0.05% 0.05% 0.05% 0.15% Patty Size 20 cm 23.7 cm 21.5 cm 25.5cm 27.5 cm Stiffening 2:15 2:35 2:15 2:55 2:30 Time

Even though more soda ash was used to obtain these results, it isconsidered to be effective because it costs one third the price of theother modifiers. Further, the soda ash increases the patty size by 37%while increasing the set time only 11%. DEQUEST 2006 yields a muchsmaller patty for about the same set time and TSPP has a smaller pattysize but has a higher set time.

EXAMPLE 3

Plant trials were conducted to test the ability of the dispersant toreduce water in wallboard made on a production line. Stucco wasdelivered to a high shear mixer via a conveyor. Dry additives, such asthe starch, were added to the conveyor as the stucco was transported.Gauge water was added to the mixer in an amount necessary to produce thetarget water to stucco ratio after water in the liquid additives wasconsidered. Trimetaphosphate was added by a separate line directly tothe mixer. All components were blended in the mixer until a homogeneousslurry was obtained. The base composition of the slurry is provided inTable III. TABLE III Base Composition, lbs. per 1000 ft² (308 m²) Stucco1714 lbs. (779 kg) Wet Gypsum Accelerator 45 lbs. (20.5 kg) Soap 0.42lbs. (19 g)

The dispersant was blended to the gauge water prior to its introductioninto the mixer at the dosages shown in Table III. The dosage shown isbased on the weight of the dry dispersant as a percentage of the drystucco. The amount of accelerator was adjusted to achieve from 50% to60% set at the knife. Since hydration is an exothermic reaction,temperature rise was used to estimate the extent of reaction and varythe amount of accelerator accordingly. As more PCE211 was used, theretardation in set resulted in a temperature at the knife indicatingless than 50% set, causing the process controller to increase theaccelerator dosage. Accelerator amounts for each data point is alsoshown in Table IV. In this table, the dispersant of this invention, PCE211 was compared to the base case of 0.14% naphthalene sulfonate and apolycarboxylate dispersant having at least three repeating units.

After all components were blended, the slurry was discharged to aconveyor containing a facing paper. A second sheet of facing paper wasplaced on top of the slurry and sent through a set of rollers to form acontinuous board of four feet (1.2 m) wide and ⅝″ (15 mm) thick. Theboard was then cut into eight foot (2.4 m) lengths at the cutting knife.TABLE IV 0.10% Dispersant 0.14% 0.21% MELFLUX 0.10% 0.125% 0.15% 0.20%Type NS NS 2500L PCE211 PCE211 PCE211 PCE211 Δ WSR Base   −0.03   −0.01 −0.01    −0.07    −0.09    −0.15 Δ Water, Base −45 −11  −21  −122 −161  −273 lbs. (kg) (−21) (−5)  (−9.5) (−56) (−73) (−124) Δ WGA, Base 0 * 2  6  6  12 lbs, (kg)  (0.9)    (2.7)    (2.7)    (5.5)* Data Not Available

Compared to the base line data containing 0.14% naphthalene sulfonate(“NS”) (DAXAD dispersant, Dow Chemicals, Midland, Mich.), changing to0.1% MELFLUX 2500L (available from Degussa) saves about 11 pounds ofwater per 1000 ft² (16 kg/1000 m²). At the same dosage, PCE211 savesabout 21 pounds of water per 1000 ft² (31 kg/1000 m²). Increasing thedosage to 0.20% allowed 273 pounds of water to be saved per 1000 ft²(886 kg/m²) of board length. However, at the highest dispersant dosage,the amount of accelerator had to be increased by 12 pounds (5.5 kg) tomeet the requirement of 50% set at the knife. Finished board propertiesat 0.5 WSR met all specifications, although core and edge hardnessresults were lower than the control sample. Bond results from certainsamples containing PCE211 were poor, but this was due to overdryingbefore the kiln temperatures were lined out.

EXAMPLE 4

To a 1 liter four necked glass flask with a temperature controller, areflux condenser and two dropping funnels 350 g water, 350 g (0.06 mol)of polyethylene glycol—5800—monovinylether and 4 g of a propyleneoxide/ethylene oxide (PO-EO)— block polymer with a molecular mass of2,000 g/mol (“defoamer”) and 25 g NaOH (20%) have been added. A mixturecomprising 45 g (0.63 mol) of acrylic acid in 17 g water has beenproduced separately and 15 g of this mixture has been added to thepolyethylene glycol—5800—monovinylether solution in the flask; the pHdecreased to 8.0. Then 40 mg iron(II)sulfate-heptahydrate (“greenvitriol”) and 3.6 g of a 50% hydrogen peroxide have been added. Within20 minutes the remaining acrylic acid mixture and 34 g of a 10%Rongalite™ solution containing 6 g of mercaptoethanol have been addedunder a constant but differing mass flow. The temperature rose from 23to 35° C. After the final addition to the reaction mixture showed a pHof 4.8. The solution has been stirred at ambient temperature for 10minute and subsequently has been neutralized with a 50 g of a 20% sodiumhydroxide solution. The product was a yellow colored, clear and aqueouspolymer solution with a solid concentration of 45% by weight.

While particular embodiments of the gypsum slurry and wallboard havebeen shown and described, it will be appreciated by those skilled in theart that changes and modifications may be made thereto without departingfrom the invention in its broader aspects and as set forth in thefollowing claims.

1. A gypsum slurry comprising: water; a hydraulic component comprisingat least 50% calcium sulfate hemihydrate by weight based on the dryweight of the hydraulic component; a defoamer; and a polycarboxylatedispersant consisting essentially of a first and a second repeatingunit, wherein said first repeating unit is an olefinic unsaturatedmono-carboxylic acid repeating unit or an ester or salt thereof, or anolefinic unsaturated sulphuric acid repeating unit or a salt thereof,and said second repeating unit is of the general formula (I)

where R¹ is represented by

and wherein R² is hydrogen or an aliphatic C₁ to C₅ hydrocarbon group,R³ is a non-substituted or substituted aryl group and preferably phenyl,and R⁴ is hydrogen or an aliphatic C₁ to C₂₀ hydrocarbon group, acycloaliphatic C₅ to C₈ hydrocarbon group, a substituted C₆ to C₁₄ arylgroup or a group conforming to the formula

wherein R⁵ and R⁷, independently of each other, represent an alkyl,aryl, aralkyl or alkylaryl group and R⁶ is a divalent alkyl, aryl,aralkyl or alkaryl group, p is 0 to 3, inclusive, m and n are,independently, an integer from 2 to 4, inclusive; x and y are,independently, integers from 55 to 350, inclusive and z is from 0 to200, inclusive.
 2. The gypsum slurry of claim 1 wherein at least one ofm and n is
 2. 3. The gypsum slurry of claim 1 wherein p is 0 or
 1. 4.The gypsum slurry of claim 1 wherein said defoaming component isselected from the group consisting of a third structural group incopolymerized form and a separated form.
 5. The gypsum slurry of claim 4wherein said defoaming component in separated form is selected from thegroup consisting of non-ionic tensides like copolymers comprisingethylene oxide/propylene oxide-(EO-PO)-units or EO-PO-EO or PO-EO-POblock copolymers and mineral oils.
 6. The gypsum slurry of claim 1further comprising at least one additive selected from the groupconsisting of set accelerators, foaming agents, trimetaphosphates,biocides, starches, sugars, siloxanes and wax emulsions.
 7. The gypsumslurry of claim 6 wherein said set accelerator comprises at least one ofwet gypsum accelerator, HRA and CSA.
 8. The gypsum slurry of claim 1wherein said first repeating unit comprises at least one of the groupconsisting of acrylic acid and methacrylic acid.
 9. The gypsum slurry ofclaim 1 wherein said hydraulic component comprises at least 80% calciumsulfate hemihydrate by weight.
 10. The gypsum slurry of claim 1 whereinsaid defoaming component is a third structural group in polymerizedform, said structural group being a propylene oxide/ethylene oxide blockcopolymer with a molecular mass of 2,000 g/mol.
 11. The gypsum slurry ofclaim 1 wherein the charge density of said dispersant is in the range ofabout 600 to about 2000 μequiv. charges/g co-polymer.
 12. The gypsumslurry of claim 1 wherein said dispersant is present in said slurry inamounts from about 0.01% to about 2% by weight of the dry dispersantcalculated as a percentage of the dry gypsum.
 13. The gypsum slurry ofclaim 12 wherein said dispersant is present in amounts from about 0.05%to about 0.3% by weight of the dry dispersant calculated as a percentageof the dry gypsum.
 14. A gypsum board comprising: a gypsum core materialcomprising calcium sulfate dihydrate; a defoamer; and a dispersantconsisting essentially of a first and a second repeating unit, whereinsaid first repeating unit is an olephinic unsaturated mono-carboxylicacid repeating unit or an ester or salt thereof, or an olephinicunsaturated sulphuric acid repeating unit or a salt thereof, and saidsecond repeating unit is of the general formula (I)

where R¹ is represented by

and wherein R² is hydrogen or an aliphatic C₁ to C₅ hydrocarbon group,R³ is a non-substituted or substituted aryl group and preferably phenyl,and R⁴ is hydrogen or an aliphatic C₁ to C₂₀ hydrocarbon group, acycloaliphatic C₅ to C₈ hydrocarbon group, a substituted C₆ to C₁₄ arylgroup or a group conforming to the formula

wherein R⁵ and R⁷, independently of each other, represent an alkyl,aryl, aralkyl or alkylaryl group and R⁶ is a divalent alkyl, aryl,aralkyl or alkaryl group, p is 0 to 3, inclusive, m and n are,independently, an integer from 2 to 4, inclusive; x and y are,independently, integers from 55 to 350, inclusive and z is from 0 to200, inclusive.
 15. The gypsum board of claim 14 wherein said corematerial further comprises at least one of the group consisting of a setaccelerator, foaming agent, set retarders, strengthening agent, starch,trimetaphosphate and a modifier.
 16. The gypsum board of claim 14wherein said board is at least 50% set within 5 minutes.
 17. The gypsumboard of claim 14 further comprising voids formed by foam.
 18. Thegypsum board of claim 14 wherein said board is at least 50% set in 10minutes in the absence of accelerators.
 19. The gypsum board of claim 14wherein the water to calcined gypsum weight ratio is less than 0.6. 20.The gypsum board of claim 19 wherein the water to calcined gypsum weightratio is less than 0.5.
 21. A gypsum slurry comprising: water; ahydraulic component comprising at least 50% calcium sulfate hemihydrateby weight based on the dry weight of the hydraulic component; and apolycarboxylate dispersant consisting essentially of a first, a secondand a third repeating unit, wherein said first repeating unit is anolefinic unsaturated mono-carboxylic acid repeating unit or an ester orsalt thereof, or an olefinic unsaturated sulphuric acid repeating unitor a salt thereof, and said second repeating unit is of the generalformula (I)

where R¹ is represented by

and wherein R² is hydrogen or an aliphatic C₁ to C₅ hydrocarbon group,R³ is a non-substituted or substituted aryl group and preferably phenyl,and R⁴ is hydrogen or an aliphatic C₁ to C₂₀ hydrocarbon group, acycloaliphatic C₅ to C₈ hydrocarbon group, a substituted C₆ to C₁₄ arylgroup or a group conforming to the formula

wherein R⁵ and R⁷, independently of each other, represent an alkyl,aryl, aralkyl or alkylaryl group and R⁶ is a divalent alkyl, aryl,aralkyl or alkaryl group, p is 0 to 3, inclusive, m and n are,independently, an integer from 2 to 4, inclusive; x and y are,independently, integers from 55 to 350, inclusive and z is from 0 to200, inclusive; and said third repeating unit is a defoaming component.22. The slurry of claim 21 wherein said defoaming component comprises atleast one of the group consisting of ester structural units,polypropylene oxide units or polyethylene oxide units.